Microwave magnetron

ABSTRACT

A crossed field electromagnetic energy device of the magnetron type is provided with integral magnetic field producing means including permanently magnetized members of a ferromagnetic material diametrically disposed adjacent to opposing sides of the anode member sidewalls together with spaced return path members. The magnetic field leakage flux is reduced by tapering the return path member walls to as close as possible to the magnet member outline. Energy leakage is controlled by eliminating axial clearance passageways for the cathode assembly found in prior art axially mounted magnet packages. A circulating fluid medium is directed between cooling fins which partially surround each magnet member and are appended to the anode member walls to control magnet material temperatures during operation and improve magnetic field efficiencies. A lighter weight device which is simple to assemble results with lower manufacturing costs as well as reduced shipping and handling costs.

4 United States Patent [191 [451 Feb. 26, 1974 Edwards MICROWAVEMAGNETRON [75] Inventor: Robert E. Edwards, Lexington,

Mass. [73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Oct. 24, 1972 211 Appl. No.: 300,066

[52] US. Cl 315/3971, 219/1055, 315/3951 [51] Int. Cl. HOlj 25/50 [58]Field of Search 315/3971, 39.51; 219/1055 [56] References Cited UNITEDSTATES PATENTS 3,493,810 2/1970 Valles 315/3971 X 3,588,563 6/1971Schmidt.. 315/3971 3,304,400 2/1967 Ojelid 315/3971 X 3,716,750 2/1973Oguro et al. 315/3971 3,588,588 6/1971 Numata 315/3953 PrimaryExaminerEli Lieberman Assistant Examiner-Saxfield Chatmon, Jr. Attorney,Agent, or Firm-Harold Murphy; Joseph D. Pannone Edgar O. Rost no n t Acrossed field electromagnetic energy device of the ABSTRACT magnetrontype is provided with integral magnetic field producing means includingpermanently magnetized members of a ferromagnetic material diametricallydisposed adjacent to opposing sides of the anode member sidewallstogether with spaced return path members. The magnetic field leakageflux is reduced by tapering the return path member walls to as close aspossible to the magnet member outline. Energy leakage is controlled byeliminating axial clearance passageways for the cathode assembly foundin prior art axially mounted magnet packages. A circulating fluid mediumis directed between cooling fins which partially surround each magnetmember and are appended to the anode member walls to control magnetmaterial temperatures during operation and improve magnetic fieldefficiencies. A lighter weight device which is simple to assembleresults with lower manufacturing costs as well as'reduced shipping andhandling costs.

2 Claims, 7 Drawing Figures PATENTEDFmsmM 'SNEEI 1 BF 4 PATENTEDrmzsmmSHEET 2 0f 4 MAGNETRON ENERGY GENERATOR ASSEMBLY ELECTRICAL CIRCUITSPATENTEU FEB 2 6 I974 sum a or 4 PRIOR ART Pmmmrzazemm SHEEI 0F 4 PRIORART LEAKAGE FLUX LINES MICROWAVE MAGNETRON BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to crossedfield electromagnetic energy devices of the magnetron type and, moreparticularly, to magnetic field producing means for such devices.

2. Descriptin of the Prior Art An electromagnetic energy generator whichhas received wide usage in the microwave field, particularly in heatingapparatus, is the magnetron. Mutally perpendicular electric and magneticfields extend within an interaction region defined between a centralcathode emitter and an anode member having a plurality ofcircumferentially disposed cavity resonators between strapped vaneelements. Such devices are energized by AC line voltages which arestepped up and rectified to DC voltages at values of, illustratively,4-6 kilovolts. The electrical field extends transverse to the axis ofthe anode member and the magnetic field extends parallel to this axis.Electrons emitted from the heated cathode are accelerated toward theanode cavity resonators and rotate in a substantially circular orbitalpath to form a rotating spoke-like space charge. The electrons interactin energy-exchanging relationship with the electric fields to generateextremely high frequency energy oscillations.

The magnetic field required for the operation of the magnetron generatoris typically provided by externally mounted electromagnets or C-shapedpermanent magents of a ferromagnetic material. The magnetron generatorcommonly generates frequencies in the microwave region of theelectromagnetic energy spectrum having wavelengths in the order of from1 meter to l millimeter and frequencies in excess of 300 MHz. The energyis coupled from the magnetron by means of an output member comprising adome of a dielectric material housing an antenna and a waveguidelaunching section coupling the energy to an enclosure. Additionaldetails relative to the magnetron energy generators may be obtained fromthe text Microwave Magnetrons, Radiation Laboratory Series, Vol. 6, byG. B. Collins,c McGraw-I-Iill Book Company, Inc., New York 19 48 Inrecent years microwave magentrons, particularly those employed forcooking applications, have utilized integral permanent magnet members toavoid the requirement of a separate voltage source for the operation ofthe prior art electromagnet structures. Magnets such as those employedin loudspeaker applications are commercially available at relatively lowcost. One such magnetron incorporates a stack of barium ferrite magnetsin either a cylindrical or a rectangular configuration axiallypositioned and surrounding the cathode external leads and supportingstructure extending from an end wall of the anode envelope. The locationof such magnetic field producing means requires axial clearaneepassageways which introduces numerous problems involving suppression ofany energy leakage over the cathode leads as well as magnetic fieldefficiencies. With the magnetic field producing means disposed at onlyone end of the device elongated and cumbersome magnetic field returnpath and shaping members must be provided. These structures includesubstantially U- shaped or tapered steel plate housings surrounding theenergy generator and external cooling fins. Examples of such prior artpermanent magnet type magnetrons are shown in US. Letters Patent Nos.3,562,579 issued Feb. 9, 1971, and 3,588,588, issued June 28, 1971 Theprovision of the efficient magnetic field strengths involvesconsideration of the leakage flux paths which extend outside of theenclosed area and represent wasted energy. Field shaping means such asbucking magnets are positioned perpendicularly to the main magnetmembers with the magnetic polarities selected to develop a repulsivefield to distort the main permanent magnet field and thereby assureproper direction within the anode-cathode interaction region.

Another problem inherent in the use of the stacked arrays of loudspeakertype magnets is the heat generated during the operation of the magnetrongenerator. The rise in the magnetic material temperature results inmagnetic circuit inefficiency unless constant cooling is provided forthe magnet members. Favorable magnetic field shaping through minimizingof the leakage flux paths, reduction of energy leakage over the externalelectrical leads and improved magnetic material cooling will result inan overall improvement in the power output efficiencies of the magnetronenergy generators under consideration.

SUMMARY OF THE INVENTION In accordance with the invention, permanentmagnet members of a ferromagnetic material are positioned as close tothe anode member sidewalls as possible on diametrically opposing sides.Magnetic field return path plate members contact both ends of the magnetmembers. The spaced plate members are provided with a tapered wallconfiguration in lieu of intersecting corners tailored as closely aspossible to the outer wall shape of the magnet walls. The reducedleakage flux paths provided with the tapered configuration results inhigh efficiency and the requirement for less ferromagnetic material forthe permanent magnet members. The placement of the magnet membersadjacent to the anode member sidewalls instead of surrounding theexternal cathode leads has also resulted in a higher magnetic circuitefficiency without the need for any bucking magnets for the shaping ofthe magnetic field. The removal of the permanent magnet members from thecathode end of the tube has obviated the need for axial passagewayswhich previously led to required suppression of any energy leakage. Theoperating temperature of the magnet material is effectively controlledand stabilized by the direction of a stream of a coolant fluid throughfins secured to the sidewalls of the anode member and partiallysurrounding the magnet members. The outermost walls of the permanentmagnet members removed from the anode sidewalls are open to provide forexposure of a larger surface area to the cooling medium.

In an exemplary embodiment of a magnetron for generation of 700 wattsoutput fewer components for cooling, magnetic field producing as well asenergy suppression were required. Additionally, the total weight wasless than 4 pounds whereas a prior art model having the permanentmagnets extending coaxially around the cathode end weighs in excess of 8pounds in one manu facturers version and approximately 7 pounds inanother manufacturers version. The lighter weight tube structure resultsin not only lower manufacturing costs but simplified assembly andreduced shipping and handling costs. Magnetron generators of the typedisclosed are also readily available at the l kilowatt level.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will bereadily understood after consideration of the following description andreference to the accompanying drawings, wherein:

FIG. I is an isometric view of the illustrative embodiment of theinvention;

FIG. 2 is an elevational view of the top magnetic field return pathmember;

FIG. 3 is an elevational view of the bottom magnetic field return pathmember;

FIG. 4 is a vertical cross-sectional view of a microwave heatingapparatus embodying the magnetron of the invention;

FIG. 5 is a partial vertical cross-sectional and partial isometric viewof a prior art permanent magnet type magnetron generator;

FIG. 6 is a vertical cross-sectional view of another prior art magneticfield producing structure as disclosed in US. Letters Patent No.3,562,579; and

FIG. 7 is a diagrammatic view of the magnetic field leakage flux pathsutilizing prior art structures shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrative embodiment ofthe invention comprising a permanent magnet type magnetron generator 10is shown in FIG. 1. Before proceeding to the description of thisembodiment, reference is directed to FIGS. 5-7 illustrating some priorart permanent magnet embodiments for such magnetron generators.

In FIG. 5 the magnetron generator 12 comprises a cylindrical conductiveanode member 14 having a plurality of circumferentially disposed cavityresonators defined between vane elements 16. Opposed conductive strapsl8 couple alternate vane elements 16 in the manner well known in themagnetron art. An axially disposed directly heated cathode 20 which maybe of the thoriated tungsten coil type is provided with end shields 22.The emitted electrons are directed in a circular orbital path withininteraction region 24 adjacent the ends of the vane elements 16. Thecathode emitter assembly is connected and supported by electrical leads26, 28 and 30 extending axially from one end of the anode member througha support structure including a conductive collar 32 and dielectrictubular member 34. The central lead 28 is connected to the upper endshield 22 and the outer leads 26 or 30 are connected to the lower endshield 22. Leads 26 or 30 and 28 provide the electrical voltages foroperation of the generator. The electrical leads are coupled to theexternal circuitry through a shielded by-pass capacitor filterarrangement housed within a box-like conductive enclosure member 36.This member is permanently secured to magnetic return path plate member38 and external leads 40 and 42 are provided with suitable clamps forconnection to the high voltage DC rectified source. The electricalenergy is coupled from the magnetron by means of a conductive antenna 44secured to one of the vane elements 16. The antenna extends within adielectric dome member 46 extending axially from the opposing side ofthe anode member. The magnetron generator is evacuated by means coupledto the dome member 46 which is then tipped off as at 418.

The magnetic field producing means includes conical-shaped inner polepiece members 50 and 52 enclosing the ends of the anode member fordirecting the magnetic field paths parallel to the anode axis within theinteraction region. The electric fields extend transverse to themagnetic field lines between the anode and cathode. A stack of low-costpermanent magnets includes a pair of large rectangular magnets 54 andsuperimposed smaller rectangular magnets 56 all supported by return pathplate member 38. The magnets are provided with axial passageways58 toaccommodate the cathode leads and member 34. Typically, for theoperation of magnetrons capable of generating 700 watts of energy amagnetic field of approximately 1800 Gauss is required. To shape themagnetic field paths within the magnetron generator interaction regionrectangular opposing bucking" magnets 60 having opposite polardesignations are supported by the U-shaped magnetic field return pathmember 62 which engages the plate member 38 and encompasses the anodemember and main magnet stack.

Cooling fins 64 contact the sidewalls of the anode member 14 fordirecting a circulating fluid medium to effectively remove the heatgenerated by the high frequency oscillations in the interaction region.It will be noted that there is no direct exposure of the magnet membersto the circulating medium forced through the fins to reduce theiroperating temperatures.

Referring next to FIGS. 6 and 7, another example of a prior artpermanent magnet arrangement is illustrated exemplifying referenced U.S.Letters Patent No. 3,562,569. In this embodiment, parallel substantiallyrectangular magnetic field return path plate members 66 and 68 supportspaced permanent magnetmembers 70 and 72. An approximation of theleakage flux paths 74 extending outside of the magnet package is shownto provide a background for the understanding of the present invention.These lines resemble a barrel around the intersecting corners and ends.It will be noted that a considerable amount of magnetic energy resultsout side of the central region 76 where the high magnetic fieldstrengths are required.

Referring next to FIG. I, the illustrative embodiment it) comprises asimilar cylindrical anode member 14 housing the components shown in FIG.5 including antenna 44 and dielectric dome member 46. The cathodesupport assembly including the external electrical leads is enclosed bya perforated boxslike member 78 which effectively suppresses any energytraversing such electrical leads. The dimension of the holes is designedto permit circulation of a coolant as well as preventing radiation ofstray energy. Within the enclosure 78 are the conventional by-passcapacitors as well as ferrite rings surrounding the cathode leads toabsorb and shunt any high frequency energy. Terminals 80 and 82 areprovided in the side of the enclosure 78 for connection of theappropriate external circuitry.

The magnetic field producing means include spaced parallel return pathplate members 84 and 86 fabricated of a rectangular body of a ferrousmaterial such as steel. The intersecting corners of the four-sided platemembers have been removed to form substantially tapered sidewalls 88tangential to the magnet outline. The tapering of walls 88, shown alsoin FIGS. 2 and 3, provides for a minimum of magnetic field leakage fluxby the removal of the ferromagnetic material which leads to theorientation of the leakage flux paths as shown in FIG. 7. The upperplate member 84 is provided with an aperture 100 to accommodate theoutput components 44 and 46 and a braided gasket 90 substantiallyprevents any stray energy leakage at this end. Holes 92 are provided formounting of the magnetron generator 12 to the waveguide transmissionline walls for coupling the energy to the microwave heating apparatus tobe hereinafter described. The enclosure 78 is secured to the'bottomplate member 86 by fastening means secured in the tapped holes 94.

Permanent magnet members 96 and 98 of a ferromagnetic material, such asAlnico, are disposed between the spaced magnetic return path members.The magnets are shaped as a single cylindrical body or a stack ofindividual magnets which can be secured together by an adhesive. Tooptimize the magnetic field paths and shape the field within the centralpassageways 100 and 102 in the desired direction parallel to the axis ofthe anode member 14, the magnets are placed close to the anode membersidewalls. To minimize the leakage flux the tapered walls 88 are formedas close as possible to the outline of the magnet members 96 and 98(shown as dashed lines in FIGS. 2 and 3) and have a length substantiallylonger than the re maining narrow straight sides of plates 84 and 86adjacent to the open ends of notches 108 in fins 104. This results inthe use of less ferromagnetic material to achieve the required magneticfield strengths due to the higher efficiency of the overall magnet andreturn path circuit.

The magnet members as well as anode member 14 operate at reducedtemperatures by the provision of cooling fins 104 stacked in an arraywith each of the fins only partially surrounding the magnet members andthereby exposing the outermost sidewalls of the magnet members to thefull thrust of a transverse flow of the circulating air indicated byarrows 106. The partial enveloping of the magnet members results from asubstantially U-shaped notch 108 in diametrically opposite sides of thecoolingfins 104. The stream of air may be provided by such means as amotor driven fan positioned adjacent to the magnetron energy generator.

The microwave heating apparatus 110 is shown in FIG. 4. Parallelconductive walls 112 define a heating enclosure 114 having an accessopening (not shown) which is closed by means of a door to permit.introduction and removal of the articles to be heated. The electricalcircuitry including timers, lights, interlock switches, as well as thehigh voltage supply has not been specifically illustrated since it isbelieved to be well known in the art and is indicated generally by thebox 116. The magnetron energy generator energized by the electriccircuits 116 has output antenna 44 disposed within a launchingrectangular waveguide section 118 adapted to couple the microwave energyfor distribution within the enclosure 114. The waveguide section 118 isshort circuited at one end by wall member 120 and is open at the innerend 122. The energy is efficiently distributed by any of the means wellknown in the art such as, illustratively, a mode stirrer 124 having aplurality of paddles 126 driven by motor 128. The articles to be heatedare supported on a lossy dielectric plate member 130 within enclosure114.

The magnetron energy generator of the invention typically operates at ananode voltage of from 4.1 4.2 kilovolts with an average anode current of300 milliamperes for the 700 watt output and 420 milliamperes for the1,000 watt power output. The overall weight of an exemplary embodimentwas 3 pounds and 9 ounces compared to a prior art model having the sameperformance characteristics of 8 pounds and 3 ounces. The new magnetronenergy generator has a considerably fewer number of parts compared tothe prior art embodiments which results in lower costs and simplifiedassembly. The magnetic field producing means have a minimum of leakageflux by reason of the tapering of the magnetic return path platemembers. The efficient cooling of the permanent magnets by the finspartially enveloping them and the full exposure of their outermostsidewalls results in more stable temperature characteristics. Strayradiation problems have also been reduced by the movement of the magnetscloser to the interaction region away from the cathode external leads.It is intended that the foregoing illustrative embodiment and detaileddescription be considered broadly since numerous variations,modifications and alterations will be readily apparent to those skilledin the art.

I claim: 1. A microwave magnetron comprising: a cylindrical anode memberdefining a plurality of circumferentially disposed cavity resonators; acathode having external electrical leads centrally disposed within saidanode member; means for producing a magnetic field parallel to thelongitudinal axis of the anode member including permanently magnetizedmembers of a ferromagnetic material disposed parallel to diametricallyopposed sidewalls of said anode member and contacting spaced rectangularreturn path plate members;

said plate members having tapered corner walls substantially longer thanthe remaining straight narrow sides; and

a plurality of cooling fin members contacting the sidewalls of saidanode member and partially surrounding said magnet members.

2. A microwave heating apparatus comprising:

conductive wall structure defining an enclosure;

a source of microwave electromagnetic energy;

means for coupling said energy from said source to energize saidenclosure;

said microwave source comprising an anode member, central cathode withexternal electrical leads, permanent magnet members disposed parallel todiametrically opposing sidewalls of said anode member and spacedrectangular return path plate members contacting opposing ends of saidmagnet members;

said plate members having tapered corner walls substantially longer thanthe remaining straight narrow sides; and

means for cooling said microwave source comprising a plurality of spacedconductive fin members appended to the anode member sidewalls andpartially surrounding said magnet members for directing the flow of afluid medium. l

1. A microwave magnetron comprising: a cylindrical anode member defininga plurality of circumferentially disposed cavity resonators; a cathodehaving external electrical leads centrally disposed within said aNodemember; means for producing a magnetic field parallel to thelongitudinal axis of the anode member including permanently magnetizedmembers of a ferromagnetic material disposed parallel to diametricallyopposed sidewalls of said anode member and contacting spaced rectangularreturn path plate members; said plate members having tapered cornerwalls substantially longer than the remaining straight narrow sides; anda plurality of cooling fin members contacting the sidewalls of saidanode member and partially surrounding said magnet members.
 2. Amicrowave heating apparatus comprising: conductive wall structuredefining an enclosure; a source of microwave electromagnetic energy;means for coupling said energy from said source to energize saidenclosure; said microwave source comprising an anode member, centralcathode with external electrical leads, permanent magnet membersdisposed parallel to diametrically opposing sidewalls of said anodemember and spaced rectangular return path plate members contactingopposing ends of said magnet members; said plate members having taperedcorner walls substantially longer than the remaining straight narrowsides; and means for cooling said microwave source comprising aplurality of spaced conductive fin members appended to the anode membersidewalls and partially surrounding said magnet members for directingthe flow of a fluid medium.